When information is missing or inconsistent, the Census Bureau uses a method called imputation to assign values. Imputation relies on the statistical principle of "homogeneity," or the tendency of households within a small geographic area to be similar in most characteristics. For example, the value of "rented" is likely to be imputed for a housing unit not reporting on owner/renter status in a neighborhood with multiunits or apartments where other respondents reported "rented" on the census questionnaire. In past censuses, when the occupancy status or the number of residents was not known for a housing unit, this information was imputed.

An operation which allows respondents to use the Census Bureau's Internet site to (1) ask questions and receive answers about the census form, job opportunities, or the purpose of the census and (2) provide responses to the short form.

Interpolation frequently is used in calculating medians or quartiles based on interval data and in approximating standard errors from tables. Linear interpolation is used to estimate values of a function between two known values. Pareto interpolation is an alternative to linear interpolation. In Pareto interpolation, the median is derived by interpolating between the logarithms of the upper and lower income limits of the median category. It is used by the Census Bureau in calculating median income within intervals wider than $2,500.

Data on language spoken at home were derived from answers to long-form questionnaire Items 11a and 11b, which were asked of a sample of the population. Data were edited to include in tabulations only the population 5 years old and over. Questions 11a and 11b referred to languages spoken at home in an effort to measure the current use of languages other than English. People who knew languages other than English but did not use them at home or who only used them elsewhere were excluded. Most people who reported speaking a language other than English at home also speak English. The questions did not permit determination of the primary or dominant language of people who spoke both English and another language. (For more information, see discussion below on "Ability to Speak English.")

Instructions to enumerators and questionnaire assistance center staff stated that a respondent should mark "Yes" in Question 11a if the person sometimes or always spoke a language other than English at home. Also, respondents were instructed not to mark "Yes" if a language other than English was spoken only at school or work, or if speaking another language was limited to a few expressions or slang of the other language. For Question 11b, respondents were instructed to print the name of the non-English language spoken at home. If the person spoke more than one language other than English, the person was to report the language spoken more often or the language learned first.

For people who indicated that they spoke a language other than English at home in Question 11a, but failed to specify the name of the language in Question 11b, the language was assigned based on the language of other speakers in the household, on the language of a person of the same Spanish origin or detailed race group living in the same or a nearby area, or of a person of the same place of birth or ancestry. In all cases where a person was assigned a non-English language, it was assumed that the language was spoken at home. People for whom a language other than English was entered in Question 11b, and for whom Question 11a was blank were assumed to speak that other language at home.

The write-in responses listed in Question 11b (specific language spoken) were optically scanned or keyed onto computer files, then coded into more than 380 detailed language categories using an automated coding system. The automated procedure compared write-in responses reported by respondents with entries in a master code list, which initially contained approximately 2,000 language names, and added variants and misspellings found in the 1990 census. Each write-in response was given a numeric code that was associated with one of the detailed categories in the dictionary. If the respondent listed more than one non-English language, only the first was coded.

The write-in responses represented the names people used for languages they speak. They may not match the names or categories used by linguists. The sets of categories used are sometimes geographic and sometimes linguistic. The following table provides an illustration of the content of the classification schemes used to present language data.

Four and Thirty-Nine Group Classifications of Census 2000 Languages Spoken at Home With Illustrative Examples

In households where one or more people (5 years old and over) speak a language other than English, the household language assigned to all household members is the non-English language spoken by the first person with a non-English language in the following order: householder, spouse, parent, sibling, child, grandchild, in-laws, other relatives, stepchild, unmarried partner, housemate or roommate, and other nonrelatives. Thus, a person who speaks only English may have a non-English household language assigned to him/her in tabulations of individuals by household language.

Some people who speak a language other than English at home may have first learned that language at school. However, these people would be expected to indicate that they spoke English "Very well." People who speak a language other than English, but do not do so at home, should have been reported as not speaking a language other than English at home.

The extreme detail in which language names were coded may give a false impression of the linguistic precision of these data. The names used by speakers of a language to identify it may reflect ethnic, geographic, or political affiliations and do not necessarily respect linguistic distinctions. The categories shown in the tabulations were chosen on a number of criteria, such as information about the number of speakers of each language that might be expected in a sample of the U.S. population.

Information on language has been collected in every census since 1890, except 1950. The comparability of data among censuses is limited by changes in question wording, by the subpopulations to whom the question was addressed, and by the detail that was published. The same question on language was asked in 1980, 1990, and Census 2000. This question on the current language spoken at home replaced the questions asked in prior censuses on mother tongue; that is, the language other than English spoken in the persons home when he or she was a child; ones first language; or the language spoken before immigrating to the United States. The censuses of 1910-1940, 1960, and 1970 included questions on mother tongue.

A change in coding procedures from 1980 to 1990 improved accuracy of coding and may have affected the number of people reported in some of the 380 plus categories. In 1980, coding clerks supplied numeric codes for the written entries on each questionnaire using a 2,000 name reference list. In 1990, written entries were keyed, then transcribed to a computer file and matched to a computer dictionary that began with the 2,000 name list. The name list was expanded as unmatched entries were referred to headquarters specialists for resolution. In Census 2000, the written entries were transcribed by "optical character recognition" (OCR), or manually keyed when the computer could not read the entry. Then all language entries were copied to a separate computer file and matched to a master code list. The code list is the master file developed from all language unique entries on the 1990 census, and included over 55,000 entries. The computerized matching ensured that identical alphabetic entries received the same code. Unmatched entries were referred to headquarters specialists for coding. In 2000, entries were reported in about 350 of the 380 categories.

Data on ability to speak English were derived from the answers to long-form questionnaire Item 11c, which was asked of a sample of the population. Respondents who reported that they spoke a language other than English in long-form questionnaire Item 11a were asked to indicate their ability to speak English in one of the following categories: "Very well," "Well," "Not well," or "Not at all."

The data on ability to speak English represent the persons own perception about his or her own ability or, because census questionnaires are usually completed by one household member, the responses may represent the perception of another household member. Respondents were not instructed on how to interpret the response categories in Question 11c.

People who reported that they spoke a language other than English at home, but whose ability to speak English was not reported, were assigned the English-language ability of a randomly selected person of the same age, Hispanic origin, nativity and year of entry, and language group.

A household in which no person 14 years old and over speaks only English and no person 14 years old and over who speaks a language other than English speaks English "Very well" is classified as "linguistically isolated." In other words, a household in which all members 14 years old and over speak a non-English language and also speak English less than "Very well" (have difficulty with English) is "linguistically isolated." All the members of a linguistically isolated household are tabulated as linguistically isolated, including members under 14 years old who may speak only English.

The current question on ability to speak English was asked for the first time in 1980. From 1890 to 1910, "Able to speak English, yes/no" was asked along with two literacy questions. In tabulations from 1980, the categories "Very well" and "Well" were combined. Data from other surveys suggested a major difference between the category "Very well" and the remaining categories. In some tabulations showing ability to speak English, people who reported that they spoke English "Very well" are presented separately from people who reported their ability to speak English as less than "Very well."